Electrolytic Process

ABSTRACT

The invention relates to a process of producing alkali metal chlorate in an electrolytic cell comprising an anode and a cathode, wherein at least one chromium compound having a valence lower than +6 is added to the process, wherein said at least one chromium compound is oxidized to hexavalent chromium within said process, wherein substantially no hexavalent chromium is added to the process from an external source. The invention also relates to the use of an aqueous solution of chromium compounds as an additive to a chlorate process.

The present invention relates to a process of producing alkali metal chlorate in an electrolytic cell comprising an anode and a cathode, wherein substantially no hexavalent chromium is added to the process from an external source.

BACKGROUND OF THE INVENTION

In the electrolysis of sodium chloride to form sodium chlorate, hexavalent chromium, usually sodium dichromate, is conventionally added to the electrolyte introduced into the cell to improve the current efficiency of the cell in the conversion of sodium chloride to sodium chlorate. This is partly obtained by suppressing the reduction of hypochlorite and chlorate at the cathode. EP 266 128 discloses a process of producing chlorate by diaphragmaless electrolysis. In this process, hexavalent chromium is added to the process from an external source.

Part of the hexavalent chromate is reduced on the cathode to Cr(III)-containing compounds such as Cr(OH)₃ and forms a very thin film which suppresses the reduction of chlorate and hypochlorite on the cathode. However, hexavalent chromium is mutagenic, reprotoxic, and carcinogenic and thus highly poisonous. There is thus a problem involved in the handling of hexavalent chromium including introduction thereof to the electrolytic cell from an external source. Therefore, strict safety precautions are a prerequisite. Hexavalent chromium also functions as a buffering solution in the chlorate electrolyte. The pH of the chlorate electrolyte may thus be liable to considerable variation in the absence of hexavalent chromium. Unfavorable pH also contributes to lower current efficiency. This may also result in undesired precipitation of compounds in the electrolyte. Hexavalent chromium thus provides for high current efficiencies in the process in several ways. The cell gas produced under optimal conditions contains a few percent of oxygen while the remaining portion of the cell gas is made up of hydrogen, but still with an oxygen to hydrogen weight ratio below the lower explosion limit. Lower current efficiency increases the oxygen/hydrogen ratio and may necessitate actions, such as dilution, to avoid explosive cell gas mixtures.

One object of the present invention is to provide an environmentally adapted process which obviates handling problems involved when introducing substantial amounts of toxic chromium(VI) compounds into an electrolytic cell from an external source while safeguarding appropriate process conditions are maintained. In particular, one object is to obviate transportation of highly toxic hexavalent chromium. A further object of the invention is to provide an alternative compound entirely substituting or to a large extent substituting toxic chromium(VI) compounds as raw material. Also, the present invention intends to provide a process which safeguards a controlled supply of hexavalent chromium in the cell electrolyte which is independent on the amount of for example hypochlorite in condensate streams. A further intention is to provide a process which facilitates production of alkali metal chlorate wherein hexavalent chromium can be provided at an acidic pH whereby necessary pH adjustment can be reduced or eliminated. A further intention of the instant invention is to provide a process which rapidly provides hexavalent chromium.

THE INVENTION

The present invention relates to a process of producing alkali metal chlorate in an electrolytic cell comprising an anode and a cathode, wherein at least one chromium compound having a valence lower than +6 is added to the process, wherein said at least one chromium compound is oxidized to hexavalent chromium within said process, and wherein substantially no hexavalent chromium is added to the process from an external source.

According to one embodiment, the process of producing alkali metal chlorate comprises introducing an electrolyte solution containing alkali metal chloride and alkali metal chlorate to an electrolytic cell, electrolyzing the electrolyte solution to produce an electrolyzed chlorate solution, transferring the electrolysed chlorate solution to a chlorate reactor to react the electrolysed chlorate solution further to produce a more concentrated alkali metal chlorate electrolyte. As the electrolysis occurs, the main anode reaction is chlorine formation and the main cathode reaction is hydrogen gas evolution and formation of hydroxide. In subsequent reactions, chlorine reacts with water and hydroxide to form a hypochlorite-hypochlorous acid mixture that in turn results in formation of alkali metal chlorate and alkali metal chloride. These reactions will start in the electrolysis cell and will continue in the downstream holding vessels. The current efficiency is less than 100% owing to a number of parasitic reactions such as anodic water oxidation leading to oxygen evolution, homogeneous hypochlorite decomposition also resulting in oxygen formation as well as cathodic reduction of chlorate and hypochlorite leading to reduced hydrogen formation. All these side reactions contribute to increasing the O₂/H₂ ratio in the cell gas.

According to one embodiment, by the wording “substantially no hexavalent chromium is added” is meant less than about 30 molar %, for example less than about 20 molar % or less than about 10 molar % or less than about 3 molar %, for example less than about 1 molar % or less than about 0.1 molar % of hexavalent chromium is added based on the total amount of chromium added to the process from an external source.

According to one embodiment, no hexavalent chromium is added to the process from an external source.

For reasons of simplicity, by the term “electrolyte solution” is meant to include the volume of all streams or solutions circulated to the electrochemical cell(s) or that will be introduced into the cell electrolyte. Examples of such liquids include, but are not limited to, alkaline scrubber solutions, brine solutions, make-up streams, process water and condensate recycled solutions. These streams include solutions of chromium compounds which do not contain any alkali chloride and/or chlorate or are alkali chloride and/or chlorate depleted electrolyte solutions.

Throughout the invention, if not otherwise stated, concentrations of anions, e.g. chloride, chlorate, hypochlorite, chromate, dichromate, sulphate, perchlorate etc, are defined as equivalent contents of their respective anhydrous sodium salts, for example NaCl, NaClO₃, NaClO, Na₂CrO₄, Na₂Cr₂O₇, Na₂SO₄ and NaClO₄.

In solution, several of these compounds participate in equilibrium reactions in ways that affect the analysis. The dichromate concentration is based on the total chromium concentration and calculated as if all chromium was in the form of sodium dichromate. Due to the equilibrium, 2 CrO₄ ²⁻+2H⁺

Cr₂O₇ ²⁻+H₂O, part of the chromium(VI) may be present as chromate. Under certain operating conditions CrO₄ ²⁻ is the dominant form.

-   In the same way, for the equilibria

HClO

H⁺+ClO⁻

Cl₂+H₂O

H³⁰+Cl⁻+HClO

the hypochlorite level stated corresponds to the hypochlorite level after transferring both Cl₂ and HClO to hypochlorite in alkaline solution by the equilibrium reactions above.

The present invention facilitates the process of providing alkali metal chlorate by excluding or minimizing the addition of highly toxic hexavalent chromium which is usually added to the aqueous chloride electrolyte solution in the form of sodium dichromate dihydrate (Na₂Cr₂O₇. 2H₂O), potassium chromate (K₂CrO₄) or mixtures thereof from an external source.

According to one embodiment, at least one chromium compound having a valence lower than +6 is added to at least one process stream containing either alkali metal chloride or alkali metal chlorate or to at least one process stream containing both alkali metal chlorate and alkali metal chlorate.

According to one embodiment, hexavalent chromium, for example in the form of sodium dichromate, can, even if added in inconsiderable amounts, be added to the process from an external source, in an aqueous solution, either separately or in combination with a chromium compound with a valence lower than +6, for example a chromium(III) compound.

In view of addition of chromium compounds, by the wording “added to the process from an external source”, as opposed to hexavalent chromium formed in the process, i.e. in-situ formation, is meant added to any process stream, for example an electrolyte stream or other process streams or to any tank, container, scrubber, reactor connected to the cell or directly to the cell. According to one embodiment, “added to the process” includes any addition point to the process from which chromium with a valence lower than +6 can be added.

According to one embodiment, a chromium compound with a valence lower than +6 is transferred from one cell line to another cell line with a compatible electrolyte composition although these may be disconnected during normal operation. An example of transfer between compatible cell lines is the transfer of electrolyte from a unit for making potassium chlorate to a unit for making sodium chlorate.

According to one embodiment, formation of hexavalent chromium is made by addition of a chromium compound with a valence lower than +6 to for example a separate medium, for example in a separate vessel, from which medium transfer of chromium compounds takes place via a process stream to the process prior to, simultaneously or subsequently to formation of hexavalent chromium. According to one embodiment, all or substantially all hexavalent chromium is formed in-situ.

According to one embodiment, the addition of chromium compounds having a valence lower than +6 may take place either during electrolysis or when the electrolysis is stopped. In particular it can take place when the starting electrolyte is prepared prior to the first start-up of a new production unit.

According to one embodiment, chromium compounds, for example dissolved in an aqueous solution, having a valence lower than +6, for example trivalent chromium can be added in a separate vessel, optionally a temporarily disconnected vessel, for example a tank, and oxidized in such vessel to hexavalent chromium (in-situ generation thereof), for example by means of hypochlorite, chlorine, chlorite, chlorate, perchlorate, chlorine dioxide, hydrogen peroxide, sodium peroxide, sodium peroxysulfate, ozone, oxygen, air or other oxidizing agent or by electrochemical anodic oxidation. Such chromium compounds can subsequently be transferred to the electrolytic cell via a process stream by pumping the chromium compound solution towards the cell.

According to one embodiment, hexavalent chromium is formed from at least one chromium compound having a valence lower than +6 by means of oxidation in an aqueous solution which hexavalent chromium is subsequently transferred to the electrolytic cell.

According to one embodiment, said at least one chromium compound with a valence lower than +6 is added in an amount resulting in a chromium content ranging from about 0.1 to about 20 g/l, for example from about 1 to about 10 or from about 2 to about 6 g (calculated as sodium dichromate equivalents)/1 electrolyte solution.

According to one embodiment, chromium compound(s) with a valence lower than +6 is/are added in an amount of from about 0.1 to about 200, for example from about 0.1 to about 100, or from about 0.1 to about 80 or from about 1 to about 60 or from about 2 to about 20 g chromium/ton produced chlorate.

According to one embodiment, addition of at least one chromium compound having a valence lower than +6, for example chromium (III), to the process may be made to the alkaline scrubber liquid and/or to the cell line loop after the cells. However, according to one embodiment, addition of at least one chromium compound having a valence lower than +6 may also be made to the electrolyte solution introduced into the cell which electrolyte solution is to be electrolyzed. According to one embodiment, the chromium compound can also be added to the electrolyzed solution prior to the reactor; to the process stream from the mother liquor scrubber; and/or to the reactor gas scrubber. According to one embodiment, the chromium compound may also be added upstream of an electrolyte filter to prevent product contamination with small amounts of possibly strongly colored insoluble chromium compounds, initially present in the chromium source or formed in the process.

According to one embodiment, chromium compounds having a valence lower than +6 may be for example chromium halides such as chromium(II)chloride, chromium(III)chloride, chromium(III)chloride hexahydrate, chromium oxide such as chromium(II)oxide (CrO), chromium(III)oxide (Cr₂O₃), chromic hydroxide, chromium(IV)oxide, chromic nitrate (Cr(NO₃)₂.9H₂O), ammonium chromate, chromic hydroxyl dichloride (Cr(OH)Cl₂), chromium sulfate pentadecahydrate, chromium sulfate, chromium hydroxide sulfate, chromium phosphate, chromite (FeCr₂O₄), or any mixtures thereof. Other suitable examples of chromium compounds are those listed in Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., Vol.6, p.526-570, 2001.

According to one embodiment, the chromium compounds can for example be added as salts, aqueous solutions or as melts if the melting point is sufficiently low, for example chromium trichloride hexahydrate having a melting point of 83° C. Solid compounds containing leachable chromium can also be used as chromium source.

According to one embodiment, chromium compounds for use may be Cr(0), for example elemental chromium, Cr(I), Cr(II), Cr(III), Cr(IV), Cr(V) or any combinations thereof. According to one embodiment, at least one Cr(III) compound is used.

According to one embodiment, the extent of electrolysis is controlled to produce an effluent from the cell in which the desired weight ratio of alkali metal chlorate to alkali metal chloride usually ranges from (expressed as a weight ratio) about 1:1 to about 20:1, for example from about 1:1 to about 15:1 or from about 2:1 to about 10:1.

According to one embodiment, the electrolyte solution may be further processed to crystallize the alkali metal chlorate such as sodium chlorate for a variety of purposes, for example for the production of chlorine dioxide for use in the bleaching of chemical cellulosic pulps, by reduction in the presence of a strong mineral acid, usually sulphuric or hydrochloric acid. Chlorine dioxide may also be generated directly from the electrolyte without prior isolation of the chlorate, typically by adding hydrochloric acid which acts both as an acid and a reducing agent.

According to one embodiment, the electrolysis produces a gaseous by-product, mainly consisting of hydrogen but also some oxygen, chlorine, hypochlorous acid, carbon dioxide and water vapour. The by-product gas stream is passed through a water condenser scrubber wherein part of the stream is condensed to form an aqueous solution of hypochlorous acid, typically about 2 to 25 g/l HOCl, which aqueous solution also contains small amounts of dissolved chlorine, which can be recirculated to the cell.

According to another embodiment, the by-product gas effluent from the water gas scrubber is optionally passed through one or several alkaline scrubbers in which chlorine and hypochlorous acid are reactively absorbed to form hypochlorite. One example is a mother liquid scrubber using the alkaline effluent from the crystallizer. Another example is a caustic scrubber using for example a NaOH solution.

According to one embodiment, the present invention is particularly directed to in-situ formation of hexavalent chromium for use in the electrolytic production of aqueous sodium chlorate from aqueous sodium chloride. However, the present invention may also be used in the electrolytic production of any aqueous alkali metal chlorate solution by the electrolysis of the corresponding chloride in which the hexavalent chromium is useful. Such aqueous alkali chlorate solutions include besides sodium chlorate also potassium chlorate, lithium chlorate, rubidium chlorate and cesium chlorate; alkaline earth metal chlorates, such as beryllium chlorate, magnesium chlorate, calcium chlorate, strontium chlorate, barium chlorate and radium chlorate, and mixtures of two or more such chlorates, which may also contain dissolved quantities of alkali metal chlorides, alkaline earth metal chlorides and mixtures thereof.

When a different alkali metal chlorate than sodium chlorate is produced the electrolyte composition and operating conditions may have to be adapted to account for differences in physical properties like solubility.

According to one embodiment, the electrolytic cell is a non-divided cell, e.g. a monopolar cell. This enables a variety of cell configurations. At least one electrode pair of anode and cathode may form a unit containing an electrolyte solution between the anode and cathode which unit may have the shape of plates or tubes. A plurality of electrode pairs may also be immersed in a cell box. According to one embodiment, the cell is a bipolar cell. A similar variety of bipolar cell configurations are also possible.

According to one embodiment, the cell is a hybrid cell, i.e. a combined monopolar and bipolar cell. This type of cells enables upgrading of monopolar technology by combining monopolar and bipolar sections in a cell-box. Such combination may be set up by arranging e.g. two or three electrodes herein as a bipolar section among a plurality of monopolar electrodes. The monopolar electrodes of the hybrid cell may be of any type including e.g. conventional electrodes known per se.

According to one embodiment, separate monopolar anodes and cathodes are mounted in an electrolytic cell at the ends, whereas bipolar electrodes are mounted in between thereby forming a hybrid electrolytic cell. According to one embodiment, the current density of the electrolytic process ranges from about 0.6 to about 4.5, for example from about 1 to about 3, or from about 1.3 to about 2.9 kA/m².

According to one embodiment, the pH is adjusted at several positions within the range from about 4 to about 12 to optimize the process conditions for the respective unit operation. Thus, a weakly acidic or neutral pH is used in the electrolyzer and in the reaction vessels to promote the reaction from hypochlorite to chlorate, while the pH in the crystallizer is alkaline to prevent gaseous hypochlorite and chlorine from being formed and released to reduce the risk of corrosion. According to one embodiment, the pH of the cell electrolyte solution, i.e. the solution comprising alkali metal chloride undergoing electrolysis in the electrochemical cell ranges from about 4 to about 7.5, for example, from about 4 to about 6.5 or from about 4 to 6 or from about 4 to 5.75 or from about 4 to 5.5. According to one embodiment, the pH of the cell electrolyte solution ranges from about 5.0 to about 7.5, such as from about 6.5 to about 7.0. According to one embodiment, the pH at the point of addition of a chromium compound having a valence lower than +6 also may range from about 4 to about 7.5, for example from about 4 to about 6.5 or from about 4 to 6 or from about 4 to 5.75 or from about 4 to 5.5. According to one embodiment, the pH of the cell electrolyte solution ranges from about 5.0 to about 7.5, such as from about 6.5 to about 7.0.

The concentration of chlorate and of chloride as well as hypochlorite in the electrolyte used in the electrochemical cell may vary widely, depending on the extent of electrolysis of the chloride solution. According to one embodiment, the electrolyte solution contains alkali metal halide, e.g. sodium chloride in a concentration from about 80 to about 180, for example from about 100 to about 140 or from about 106 to about 125 g/l electrolyte. According to one embodiment, the electrolyte solution contains alkali metal chlorate in a concentration from about 200 to about 700, e.g. from about 450 to about 650 or from about 550 to about 610 g/l. According to one embodiment, the concentration of hypochlorite in the electrolyte solution ranges from about 0 to about 6, for example from about 0.01 to about 4 or from about 0.1 to about 4 or from about 0.3 to about 3 g/l. The electrolyte may also comprise significant amounts of inactive compounds accumulated in the process over the course of time, for example sodium sulfate introduced as an impurity in the sodium chloride source or sodium perchlorate formed by a side reaction in the process.

According to one embodiment, the weight ratio of chromium derived from chromium compound(s) having a valence lower than +6 added to the process to hypochlorite ranges from about 1:30 to about 3:1, for example from about 1:10 to about 2:1, or from about 1:8 to about 1:1.

According to one embodiment, the amount of hexavalent chromium formed from the chromium compound having a valence lower than +6 is in the range from about 0.1 to about 25 grams calculated as sodium dichromate ions/l electrolyte solution in the cells, for example from about 0.2 to about 15 g/l electrolyte solution, for example from about 1 to about 8 g/l electrolyte solution. In order to arrive at such suitable amount of hexavalent chromium, a corresponding amount of chromium compound with a valence lower than +6 is added.

According to one embodiment, the flow to the chlorate cells normally is from 75 to 200 m³ of electrolyte per metric ton of alkali metal chlorate produced. According to one embodiment, the electrolytic cell operates at a temperature ranging from about 60 to about 100° C., or from about 65 to about 90° C. According to one embodiment, the temperature of the stream or solution at the addition point of a chromium compound having a valence lower than +6 ranges from about 60 to about 100° C., or from about 65 to about 90° C. According to one embodiment, the temperature of the stream or solution at the addition point of a chromium compound having a valence lower than +6 ranges from about 15 to about 40° C., for example from about 15 to 30° C. According to one embodiment, a part of the electrolyzed solution is recycled from the reaction vessels to a salt dissolver, and a part is recycled for alkalization and electrolyte filtration and final pH adjustment before introduction into the chlorate crystallizer. Water from the thus alkalized electrolyte can be evaporated in the crystallizer. According to one embodiment, the mother liquor, which is saturated with respect to chlorate and contains high contents of sodium chloride, is recycled directly to the preparation slurry via cell gas scrubbers and reactor gas scrubbers. According to one embodiment, the pressure in the cell is about 20 to 30 mbar above the atmospheric pressure.

According to one embodiment, the (electrical) conductivity in the cell electrolyte ranges from about 200 to about 700, for example from about 300 to about 600 mS/cm.

According to one embodiment, the electrolytic cell and the electrodes arranged therein may be as further disclosed in EP 1242654 and WO 2009/063031.

The invention also relates to the use of an aqueous solution or a melt of chromium compounds as an additive to a chlorate process, said solution comprising at least one hexavalent chromium compound and at least one chromium compound having a valence lower than +6, wherein the molar ratio of hexavalent chromium to chromium having a valence lower than +6 ranges from about 1:10000 to about 3:10, for example from about 1:10000 to about 2:10, for example from about 1:10000 to about 1:10, for example from about 1:10000 to about 1:100 or from about 1:10000 to about 1:1000, or from about 0:10000 to about 1:10000. According to one embodiment, the molar ratio of hexavalent chromium to chromium having a valence lower than +6 ranges from about 0:10000 to about 1:10000.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the gist and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the claims. The following examples will further illustrate how the described invention may be performed without limiting the scope of it. If not otherwise specified, all percentages given herein concern percent by weight.

EXAMPLE 1

Dissolution of chromium trichloride hexahydrate in acidic, neutral and alkaline aqueous solutions, was performed.

Four beakers 1-4 with 100 ml aqueous solution each were prepared at room temperature (295 K) according to the below:

-   -   1. 0.05 M HCl     -   2. Deionized (DI) water     -   3. 0.05 M NaOH     -   4. Chromium free chlorate electrolyte (110 g/l NaCl, 550 g/l         NaClO₃ in water, no pH adjustment).

To each beaker, 22 mmol of CrCl₃*6H₂O was added and the opening was covered with parafilm. The solutions were left standing for 72 hours at room temperature.

TABLE 1 Observations during solubility tests of Chromium(III) chloride in different solvents Beaker/solvent 1 2 3 4 0.05M HCl DI water 0.05M NaOH Cr free electrolyte 0 h All All Precipitation All dissolved, dissolved. dissolved. observed. but slower to Green Dark green Grayish-green dissolve. Green 5 h Emerald green Bluish green Bluish green Green 72 h  Emerald green Blue to purple Bluish green. Bluish green. pH 1.42 pH 3.03 Grey pH 1.92 precipitate at the bottom. pH 4.97

When the mixture in beaker 3, with the precipitate, was acidified with 2 M HCl to a pH of 1.90 the precipitate was dissolved within 48 h. The colors indicate that chromium remained trivalent in all cases since trivalent chromium species varies in color from green to violet blue (dark green [CrCl₂(H₂O)₄]Cl, pale green [CrCl(H₂O)₅]Cl₂, and violet [Cr(H₂O)₆]Cl₃)) and hexavalent chromium varies from orange (dichromate) to yellow (chromate).

EXAMPLE 2

1.5 dm³ electrolyte containing 120 g/l NaCl and 580 g/l NaClO₃ was prepared by dissolution at 90° C. in water and dilution to 1.5 dm³. To the warm electrolyte 6 g/l of CrCl₃*6H₂O was added. The color was at first bluish green but darkened as gas started to evolve. After a few seconds a substantial amount of yellow gas was formed. 3 minutes later the gas evolution ceased and the color of the electrolyte became dark orange. This demonstrated that dichromate had formed by oxidation of Cr(III) by means of chlorate which simultaneously had been reduced to ClO₂(g) in the acidic solution.

EXAMPLE 3

100 ml of chromium free electrolyte containing 110 g/l NaCl and 550 g/l NaClO₃ was prepared at room temperature. 22 mmol CrCl₃*6H₂O was added and the solution was placed on a heater equipped with magnetic stirrer and heated to 95° C. A glass thermometer was placed in the beaker to monitor the temperature. The pH after Cr(III) addition was 1.92.

TABLE 2 Observation during heating of Cr(III) containing electrolyte with a pH of 1.92 Cr oxidation state Temperature Observations based on color 22° C. Bluish Green III 60° C. Emerald green III 67° C. Indications of yellowing III + VI lighter green 85° C. Moss green III + VI 90° C. Brownish yellow. Gas Mainly VI bubbles appear 95° C. Orange VI At 95° C., essentially all chromium(III) had been oxidized to chromium(VI). The time required to raise the temperature from 22° C. to 95° C. was approximately 20 min.

EXAMPLE 4a

50 ml of synthetic electrolyte (110 g/l NaCl, 550 g/l NaClO₃ and 3 g/l Na₂Cr₂O₇) was prepared and the pH was adjusted to 6.90 with NaOH(s). When the electrolyte had been cooled down from 90° C. to 40° C., 0.22 mmol of CrCl₃*6H₂O crystals was added to the electrolyte and a brown precipitation was formed. The pH after addition of the CrCl₃*6H₂O crystals was 5.80. The mixture was placed on a heater with a magnetic stirrer and a glass thermometer was placed in the beaker. At 88° C., minor gas evolution occurred. Even as the temperature reached 100° C. the precipitation remained. Thus, no oxidation took place due to the high pH.

EXAMPLE 4b

50 ml of synthetic electrolyte (110 g/l NaCl, 550 g/l NaClO₃ and 3 g/l Na₂Cr₂O₇) was prepared and the pH was adjusted to 6.40 with NaOH(s). It was then heated to 85° C. while stirring. A small amount of CrCl₃*6H₂O crystals (˜0.05 g) was added and a brown precipitation was formed. To this electrolyte 0.2 ml of sodium hypochlorite (˜150 g/l in 0.5 M NaOH) was added whereby the precipitation was dissolved. The remaining electrolyte was yellow and had a pH of 6.00. The hypochlorite had oxidized all chromium (III) to chromium (VI).

EXAMPLE 5

Chlorate electrolyte was withdrawn from the electrolysis cell outlet of a chlorate plant during operation. The addition of a small amount of CrCl₃*6H₂O crystals (˜0.1 g/100 ml electrolyte) resulted in complete dissolution and oxidation of chromium (III) to chromium (VI). A larger amount of CrCl₃*6H₂O (˜0.5 g/100 ml) resulted in formation of a brown precipitation whereby no further oxidation to hexavalent chromium took place.

EXAMPLE 6

A hypochlorite containing caustic scrubber solution was withdrawn from a chlorate plant and CrCl₃*6H₂O crystals were added (˜0.1 g/100 ml electrolyte). The CrCl₃*6H₂O crystals first dissolved forming a green solution and later oxidized forming a pale yellow Cr(VI) containing solution.

In view of the above examples, it can be concluded CrCl₃*6H₂O crystals can be easily dissolved and that pH is reduced on dissolution of acidic CrCl₃*6H₂O. Precipitation occurred close to neutral pH and in weakly alkaline solutions, presumably due to formation of Cr(OH)₃(s) or CrO₂(s). Sodium chlorate has been found to oxidize chromium (III) to chromium (VI) under acidic conditions and at elevated temperatures while chlorine dioxide is formed which can be recovered by absorption in alkaline electrolyte or caustic, whereby it forms chlorate, chlorite and/or chloride.

Sodium hypochlorite can oxidize chromium(III) to chromium(VI) in strongly alkaline solutions and down to at least pH 5.8, for example to at least pH 5 or below pH 5. Hypochlorite can even dissolve precipitations formed in neutral solutions and oxidize chromium with a valence lower than +6 to hexavalent state.

These examples demonstrate that trivalent chromium but also other valencies of chromium lower than +6 are a viable alternative to hexavalent chromium as raw material in a process for the production of alkali metal chlorate since it is easily oxidized to the hexavalent state; either by chlorate or hypochlorite oxidation. Addition of a chromium compound, for example chromium(III), to the process may be made for example in the scrubber caustic solution, in the cell line loop after the cells, or to the inlet of the cell.

EXAMPLE 7

A 203 ml (conventional dichromate-containing) electrolyte composition containing 110 g/dm³ NaCl, 550 g/dm³ NaClO₃, 5.0 g/dm³ Na₂Cr₂O₇ was used in trials conducted at a pH of 6.1 and a temperature of 25° C.

A hypochlorite solution of 2 g (NaClO content was 124 g/dm³) was added to the electrolyte and subsequently 0.4788 g of a 50 wt % solution of Cr(III)Cl₃×6H₂O. It could be noted a change of colour occurred after addition of the solution of Cr(III)Cl₃×6H₂O such that the electrolyte initially turned darker but after a while turned yellow again as Cr(III) oxidized to Cr(VI).

EXAMPLE 8

A 261.28 g (conventional dichromate containing electrolyte) composition containing 110 g/dm³ NaCl, 550 g/dm³ NaClO₃, 5.0 g/dm³ Na₂Cr₂O₇ was used in trials conducted at a pH of 6.1 and a temperature of 25° C. 0.4815 g of a 50 wt % solution of Cr(III)Cl₃×6H₂O was added thereto and subsequently a hypochlorite solution of 2 g (NaClO content was 124 g/dm³).

It could be noted the solution started to change colour subsequent to addition of hypochlorite solution indicating oxidation of Cr(III) to Cr(VI). 

1. Process of producing alkali metal chlorate in an electrolytic cell comprising an anode and a cathode, wherein at least one chromium compound having a valence lower than +6 is added to the process in an amount from 1 to 200 g chromium/ton produced chlorate, wherein said at least one chromium compound is oxidized to hexavalent chromium within said process, wherein substantially no hexavalent chromium is added to the process from an external source.
 2. The process according to claim 1, wherein the amount of hexavalent chromium added is less than about 30 molar percent based on the total amount of chromium added from an external source.
 3. The process according to claim 1, wherein no hexavalent chromium is added.
 4. The process according to claim 1, wherein a chromium(III) compound is added to the process.
 5. The process according to claim 1, wherein said at least one chromium compound having a valence lower than +6 is added to the scrubber liquid, to the cell line loop after the cells, or to the electrolyte going in to the cell.
 6. The process according to claim 1, wherein said at least one chromium compound having a valence lower than +6 is added to the electrolyzed solution prior to the reactor; to the process stream from the mother liquor scrubber; and/or to the reactor gas scrubber.
 7. The process according to claim 1, wherein said at least one chromium compound having a valence lower than +6 is added upstream of an electrolyte filter.
 8. The process according to claim 1, wherein said at least one chromium compound having a valence lower than +6 is added in an amount resulting in a chromium content from about 0.1 to about 20 g (calculated as sodium dichromate equivalents)/l electrolyte solution.
 9. The process according to claim 1, wherein the weight ratio of chromium derived from chromium compounds having a valence lower than +6 to hypochlorite ranges from about 1:30 to about 3:1.
 10. The process according to claim 1, wherein the amount of chromium compound(s) with a valence lower than +6 is added in an amount from about 1 to about 60 g chromium/ton produced chlorate.
 11. The process according to claim 1, wherein hexavalent chromium is formed from at least one chromium compound by means of oxidation in an aqueous solution which hexavalent chromium is subsequently transferred to the electrolytic cell.
 12. The process according to claim 11, wherein hexavalent chromium is formed in an aqueous solution in a medium separated from the process prior to transfer of said aqueous solution of hexavalent chromium to the process.
 13. The process according to claim 1, wherein substantially all hexavalent chromium within the process has been formed in-situ.
 14. A process for producing alkali metal chlorate in an electrolytic cell comprising an anode and a cathode, the process comprising adding an aqueous solution of chromium compounds to the chlorate process, said solution comprising chromium compounds, wherein the molar ratio of hexavalent chromium to chromium having a valence lower than +6 ranges from about 0:10000 to about 1:10000.
 15. A process for producing alkali metal chlorate in an electrolytic cell comprising an anode and a cathode, the process comprising adding an aqueous solution of chromium compounds to the chlorate process, said solution comprising at least one hexavalent chromium compound and at least one chromium compound having a valence lower than +6, wherein the molar ratio of hexavalent chromium to chromium having a valence lower than +6 ranges from about 1:10000 to about 3:10.
 16. The process according to claim 2, wherein a chromium(III) compound is added to the process.
 17. The process according to claim 3, wherein a chromium(III) compound is added to the process.
 18. The process according to claim 14, wherein the at least one chromium compound having a valence lower than +6 is a chromium(III) compound.
 19. The process according to claim 15, wherein the at least one chromium compound having a valence lower than +6 is a chromium(III) compound. 